Crabs are generally covered with a thick exoskeleton, and armed with a single pair of chelae (claws). Crabs are found in all of the worldâ€™s oceans, while many crabs live in fresh water and on land, particularly in tropical regions. Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span of up to 4 meters (13 ft).

About 850 species of crab are freshwater, terrestrial or semi-terrestrial species; they are found throughout the worldâ€™s tropical and semi-tropical regions. Crabs typically walk sideways. This is because of the articulation of the legs which makes a sidelong gait more efficient. However, some crabs prefer to walk forwards or backwards, including raninids, Libinia emarginata and Mictyris platycheles. Some crabs, notably the Portunidae and Matutidae, are also capable of swimming.

Crabs are mostly active animals with complex behavior patterns. They can communicate by drumming or waving their pincers. Crabs tend to be aggressive towards one another and males often fight to gain access to females. On rocky seashores, where nearly all caves and crevices are occupied, crabs may also fight over hiding holes.

Crabs are omnivores, feeding primarily on algae, and taking any other food, including molluscs, worms, other crustaceans, fungi, bacteria and detritus, depending on their availability and the crab species. For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness. Crabs are known to work together to provide food and protection for their family, and during mating season to find a comfortable spot for the female to release her eggs.

Crabs make up 20% of all marine crustaceans caught, farmed, and consumed worldwide, amounting to 1Â½ million tonnes annually. One species accounts for one fifth of that total: Portunus trituberculatus. Other commercially important taxa include Portunus pelagicus, several species in the genus Chionoecetes, the blue crab, Charybdis spp., Cancer pagurus, the Dungeness crab and Scylla serrata, each of which yields more than 20,000 tonnes annually.

Crabs are prepared and eaten as a dish in several different ways all over the world. Some species are eaten whole, including the shell, such as soft-shell crab; with other species just the claws and/or legs are eaten. The latter is particularly common for larger crabs, such as the snow crab. In Asia, masala crab and chilli crab are heavily spiced dishes. In Maryland, blue crab is often eaten with Old Bay Seasoning. For the British dish Cromer crab, the crab meat is extracted and placed inside the hard shell. One American way to prepare crab meat is by extracting it and adding a flour mix, creating a crab cake.

In 2005, Norwegian scientists concluded that lobsters cannot feel pain. However, later research suggests that crustaceans are indeed able to feel and remember pain.